Many people say training to failure is good for muscle building and others say the oppoisite.
What are your views on it? Is it good or bad and the reasons why, too please.

Some say that it is too hard on the system, and that it could easily lead to overtraining. I say that the GPO principle applies: Gradual Progressive Overload, key word being Overload. Overload is working the muscles beyond what they are used to (what they have already compensated for). If you don't take a set to failure, I tend to think that you're not recruiting enough muscle fiber to optimally overload the muscle and have it overcompensate with hypertrophy. Now, let's back up a second and look at the first word: Gradual. Of course, gradual means nothing dramatic, like 15 sets to complete failure and beyond for each bodypart; that would likely push someone into a state of overtraining, unless they're not natural, and have good genetics.

Some advocate not going to failure and increasing the volume (versus a strict Heavy Duty practitioner, who may do a full-body workout, 1 set per bodypart, taking to complete failure). Volume does increase load, and not going to failure admittedly decreases the risk of injury and overtraining, but you are also doing more physical work, due to more sets.

Bottom line, in my meaningless opinion: exercise science is not an exact science, and people differ in their physiological responses to training and their recuperative ability. Add to this the fact that many great physiques have been built with many diffferent approaches, and I think most can agree that there is no one perfect 'cookie cutter' routine for everyone.

I have a question about training to failure, say your told to do a certain amount of reps (8-12, lets say). If you say squat 200lbs for ten reps and fail, do you decrease the weight for your second set (because if you didn't you would only be able to do 6-7 reps before failing) or do you squat 200 lbs again on your second set?